In these lessons, you'll
animate a four-legged character, a dog, to walk in a continuous
way. You’ll use the ForeFeet option to make the fingers of the biped
hands behave like toes on forefeet.

A Quick Review of a Biped
Walk

If you don’t use Biped
to create a walk for you, it helps to know that a human walk cycle
is defined by two steps: left foot to right foot, followed by right
foot to left foot (or vice versa). The two steps break down into
four states:

Left to right:

1. Contact

2. Down

3. Passing

4. Up

5. Contact again (same
as 1, but with legs reversed)

Contact: Both
feet are on the ground. At this point, the stride is at its longest:
this is known as an extreme pose.

Down or “Recoil”: After
contact, the weight goes down on the front leg. The body lowers,
and both legs bend.

Passing or “Breakdown”: The
front leg straightens and the back leg passes it. The body raises
to a point that is higher than in the contact position.

Up or “High Point”: The
back foot is now the front one, and is about to make contact. The
other foot pushes up and forward, raising the body to its highest
position.

Contact: The
same as pose 1, but with the opposite leg forward.

You can start animating
the cycle at any of these poses. Animators often prefer to begin
with the contact pose, as that pose (in general, any extreme pose)
is a good reference to build from.

You have to decide how
many frames the walk cycle will use. 12 frames yields two steps
per second: this is a natural pace, which we will use in this tutorial.
Cartoonists sometimes use an 8-frame cycle to create a fast, humorous
walk. A 24-frame cycle would give (for film) one step per second, suitable
for a slow-moving character.

The Walk Cycle for Quadrupeds

A quadruped walk is essentially
two biped walks linked together, but out of phase with each other.
When a biped walks, the shifting weight on the pelvis causes the
up-down motion just described. For a quadruped, the same weight
shifts occur for the pelvis and the shoulders.

Quadrupeds have different
proportions than human bipeds. In particular: